Governmental restrictions on the load which may be carried by each axle of a dump truck, and other considerations regarding the need to optimize the locations of load bearing axles of such trucks, has led to the use of tag wheel assemblies with such trucks. These assemblies comprise wheels affixed to the truck by means of a rigid bridge which is pivotally connected to the truck and is hydraulically driven from an operative position wherein the wheels of the assembly engage the supporting surface to carry a portion of the load on the truck. to an inoperative position with the bridge and wheels extending upwardly out of the way to permit thee contents of the truck to be dumped where desired.
It is important that the point of pivotal connection of the bridge to the truck frame be positioned as close to the rear end of the truck bed as possible for several reasons. The truck must be able to dump its contents into the hoppers of lay down machines (machines which apply or "lay" road paving materials on the road bed in the construction or refurbishing of streets and highways). Most hoppers of lay down machines are not very long. This requires that the dump truck tailgate be moved relatively close to the lay down machine to effectively dump the truck contents into the hopper. If the tag wheel bridge is positioned a substantial distance rearwardly of the rear end of the truck bed, it will interfere with the positioning of the truck for this purpose.
Additionally, the effective loading and proper positioning of the truck axles is adversely affected if the tag wheel bridge pivot is too far from the rear of the truck. The optimum weight-balance relationship for the truck dictates that this pivot point be as closely spaced to the rear of the truck bed as is structurally possible.
However, if the tag wheel pivot is close to the truck bed, the upright assembly in the standby position interferes with the conventional truck tailgate when the bed is hoisted and the tailgate is raised during the dumping operation. One solution to this problem is shown and described in U.S. Pat. No. 4,940,287 to Ritchie, issued Jul. 10, 1990. The twin tailgates construction with a fixed deflector between the gates disclosed in that patent is workable, and that system has allowed tag wheels to be used in conjunction with dump trucks successfully. However, some disadvantages have been experienced with that solution to the problem. The double tailgate construction is somewhat more costly than that of a single tailgate and some interference with complete emptying of materials from the truck bed has been reported. The deflector or tunnel between the twin tailgates can also retard the flow of some materials from the truck.
One approach which has been tried is to pivot the tag wheel assembly a substantial distance to the rear of the bed to provide room for the swinging of the tailgate and the dumping of the materials between the rear of the bed and the pivot point. This approach has the disadvantages of improper weight distribution on the axles resulting from the large distance between the pivot point and the bed as discussed above. The structure forward of the pivot point which projects rearwardly from the truck also prevents the truck from backing closely to lay down machine hoppers and the like as heretofore discussed.
Positioning the tag wheel assembly so that it is relatively close to the dump bed is therefore highly desirable, but this puts the assembly in position to interfere with the opening of conventional tailgates as required for dumping. Accordingly, a need has arisen for a tailgate which is constructed to operate in a manner to accommodate the close-in position of the tag wheel assembly and yet permit substantially unrestricted dumping of the truck bed.